1. Field of the Invention
The present invention generally relates to electronic circuits and, more specifically, to at least partially digital electronic circuits responsive to flip-flop states.
The present invention more specifically applies to the detection of a modification in the state of a flip-flop between two updating times.
2. Discussion of the Related Art
Logic states processed by a digital circuit may undergo incidental or forced disturbances. A forced disturbance comprises, for example, locally disturbing the circuit operation (for example, by means of a laser) to force one or several logic elements (typically flip-flops) to switch state. Such disturbances are generally called “fault injections”. The results of fault injection attacks are exploited by analysis mechanisms, for example of the type known as DFA (Differential Fault Analysis).
Disturbing the state provided by a logic element of a flip-flop type may generate a more general circuit malfunction. For example, this may disturb the operation of a cryptography algorithm to discover manipulated secret quantities. According to another example, this may enable the circuit to set to a state which is in principle unauthorized (for example, to the test mode).
Flip-flop malfunctions may be of dynamic or static nature. Dynamic malfunctions result from shifts in the clock signal (glitches) or from modifications in the clock tree. Static malfunctions, to which the present invention applies, are, for example, generated by laser-type attacks to cause an output state switching of one or several flip-flops without requiring a trigger signal edge to take this state switching into account.
It would be desirable to be able to detect a possible abnormal state switching of one or several flip-flops in an integrated circuit, be this state switching incidental or forced.